As far as I know sun works too. Exposure time has to be considerably longer though and ... well ... the sun should actually be out. Pacific North West in winter is probably not working so well. Australian desert in the summer should work well ;)

This method is approved by the World Health Organization for treating household water. Here's a link:

http://www.sodis.ch/methode/index_EN

For the lazy, here's a link to some 30+ studies that confirm light filtering through clear bottles (like PET plastic) will disinfect water a significant amount over 6 hours: http://www.sodis.ch/methode/forschung/publikationen/index_EN#health

Edit: Here's some more info.

-Even extreme levels of fecal matter (100,000 E. Coli per 100ml) are rendered harmless by UV exposure using the SODIS method (Solar Disinfection).-A bottle 3 liters or smaller must be placed on its side for 6 hours in direct sunlight to be rendered harmless.-Only clear bottles will correctly allow UV radiation in.-Some bacteria grows with sunlight, but this is harmless bacteria.

Max beat me to it. Google "SODIS". There are caveats. Due to required exposure time even in ideal environments I would consider SODIS an emergency/survival tactic. Not super useful for the typical BPL hiker but it could be a lifesaver if you're on an international trek near the equator and run into water treatment problems.

Reading through the link from above, it seems that 6ish hours are required. Probably not practical for backpacking but great for third world countries though. I previously understood that UVC was the requirement for sterilizing pathogens but there's obviously more to the story than that. Seems temperature plays into the SODIS process as well.

I went to a global health conference a while ago and attended a talk about SODIS implementation in Africa. A major obstacle to acceptance by people living in African villages is stigma and aesthetics. In areas where waterborne parasites cause high mortality among children, people who adopt the SODIS method for water disinfection are stigmatized and ridiculed by their neighbors. People consider them gullible for believing that the water is unsafe just because some health worker told them so. They also don't like the way the bottles look, lined up on the roof.

Anyway, I would have to agree with the other posters: it doesn't seem practical for backpacking except on perfectly clear days at high elevation or low latitude. And the bottle(s) need to be thin-walled PET (not thick-walled lexan Nalgene) and strapped on the pack in full sun (no part shaded).